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Abstract:

The laughing-gas-containing gas (1) is diluted by means of a diluting gas
(2). The diluting gas (2) is virtually free from water fractions in the
dryer (3). After feed of the diluting gas (2) via the feed line (13),
exhaust gas (8) from the catalytic decomposition (7) is added (4) to the
laughing-gas-containing feed gas (12). After addition (4) of the exhaust
gas (8) from the catalytic decomposition of laughing gas (7), the
laughing-gas-containing feed gas (12) is compressed (5) and passed to the
heat exchanger (6). In the heat exchanger (6) the laughing-gas-containing
feed gas (12) is preheated by heat exchange with the exhaust gas (8). The
exhaust gas (8) is cooled in the heat exchanger (6) in this process. The
preheated laughing-gas-containing feed gas (12) is passed via a further
optional heater (11) as a feed to the catalytic laughing gas
decomposition (7). In order to avoid a concentration build-up, some of
the exhaust gas (8) is passed out (9) of the process. The exhaust gas (8)
is further cooled by a further optional heat exchanger (10). The
additional cooling by means of the heat exchanger (10) ensures that
subsequent appliances such as the compressor are protected against
overheating. By means of the heat exchanger (10), the permissible
temperature of that part of the exhaust gas (8) which is released (9) to
the atmosphere can also be controlled here. For optimal control of the
intake temperature of the laughing-gas-containing feed gas (12) into the
catalytic decomposition (7), at least some of the laughing-gas-containing
feed gas (12) can bypass the heat exchanger (6) using a bypass (14).

Claims:

1. A method for catalytic decomposition of laughing gas in a
laughing-gas-bearing gas, wherein the laughing-gas-bearing gas is diluted
with a diluting gas, while forming a laughing-gas-bearing charge gas and
is passed through a heat-exchange step during heat exchange with an
exhaust and a heating step for occasional heating of the
laughing-gas-bearing charge gas in a fixed-bed reactor for catalytic
decomposition of the laughing has, characterized in that the diluting
gas, which is added to the laughing-gas-bearing gas and is dried, and at
least a part of the exhaust from the catalytic decomposition of the
laughing gas is mixed with the laughing-gas-bearing charge gas upstream
of the catalytic decomposition of the laughing gas.

2. A method according to claim 1, characterized in that air or an inert
gas is used as a diluting gas.

3. A method according to claim 1, characterized in that the diluting gas
is dried so that the laughing-gas-bearing charge gas exhibits a dew point
of -20.degree. C.

4. A method according to claim 1, characterized in that the mixing of the
exhaust into the laughing-gas-bearing charge gas takes place upstream of
the heat-exchange step and of heat exchange with the exhaust.

5. A method according to claim 1, characterized in that drying of the
diluting gas takes place with a single-pass dryer, a molecular sieve, an
adsorbent, an exchange-bed adsorbent, or a diaphragm dryer.

6. A method according to claim 1, characterized in that at least a part
of the exhaust is passed directly into the laughing-gas-bearing charge
gas.

7. A method according to claim 1, characterized in that several
laughing-gas-bearing gas streams are collected in a gas receptacle and
are removed to it as a laughing-gas-bearing gas.

8. A device for catalytic decomposition of laughing gas comprising a
supply of the laughing-gas-bearing charge gas to a compressor, a heat
exchanger, a heater, and a fixed-bed reactor which exhibits a catalyst
and is suitable for catalytic decomposition of laughing gas, as well as
piping in the supply for the compressor, characterized in that the device
exhibits an engineered flow connection between the exhaust outlet of the
fixed-bed reactor and the supply to the compressor, which leads to the
heat exchanger, and the piping is connected to a dryer suitable for
drying a diluting gas.

9. A method according to claim 1, characterized in that the diluting gas
is dried so that the laughing-gas-bearing charge gas exhibits a dew point
of -40.degree. C.

Description:

[0001] The invention concerns a method for catalytic decomposition of
laughing gas in a laughing-gas-bearing gas, wherein the
laughing-gas-bearing gas is diluted with a diluting gas in forming a
laughing-gas-bearing charge gas and is passed through a heat-exchange
step in heat exchange with an exhaust and a heating step for the
occasional heating of the laughing-gas-bearing charge gas into a
fixed-bed reactor for catalytic decomposition of laughing gas, as well as
a device for carrying out the method. Such a method and such a device are
known, for example, from the US patent 72335222, US patent application
2006/0008401, or WO 2006/059506.

[0002] Nitrous oxide or laughing gas is a non-poisonous gas, which has
long been used as an anesthetic or analgesic agent alone or in
combination with other substances. In addition, laughing gas also
originates as a by-product in the manufacture of nitric acid or is used
in organic syntheses. A problem exists in application as an anesthetic or
analgesic agent, in that the laughing gas exhaled by the patient can be
concentrated in the treatment space above the current maximally
permissible workplace concentration. Such concentrated laughing gas can
lead over a long time to an excessive burden for medical personnel.

[0003] Additionally, laughing gas also contributes in a considerable
amount to the greenhouse effect and to breakdown of the ozone layer.
Hence from the point of view of environmental technology, decomposition
of laughing gas is necessary before release into the atmosphere.

[0004] There is in prior art a demand for a method of effective
decomposition of laughing gas from various sources, be it from medical
treatments, nitric-acid production, exhausts from various organic
syntheses, in the cleaning/evacuation of gas cylinders, or from exhaust
streams in the very manufacture of laughing gas.

[0005] The patents and patent applications cited above deal with the
recovery of laughing gas from medical applications and essentially
propose collecting the laughing gas that accumulates in several treatment
or operating spaces of a hospital complex and purifying in a common
catalytic treatment. Catalysts are preferably used for this which make
decomposition of the laughing gas possible at relatively low
temperatures, that is, below 600° C. Different adsorbents in the
catalytic treatment can be connected in series in order to separate out
disruptive components such as sevofluorane, desfluorane, isofluorane,
halothane, or other halogenated hydrocarbons.

[0006] In addition, a method and a device for decomposition of laughing
gas has been proposed by the inventors of the present application in the
unpublished European patent application EP 08164753.9, which provides for
transferring laughing gas accumulated directly on site in the treatment
room for an individual patient, and thus avoids expensive collection of
the different streams of laughing gas within the hospital complex.

[0007] Commercially available methods for laughing-gas removal in prior
art are based mainly on thermal or catalytic decomposition of laughing
gas at more than 800° C.

[0008] More modern catalytic methods, which operate at temperatures below
600° C., are a common problem, namely that the decomposition of
laughing gas is highly exothermic. This means that if the gas exhaled by
the patient is subject to no further pre-processing for catalytic
decomposition, then, due to the high laughing-gas concentration of up to
70%, such a high exothermic reaction or heat development at the catalyst
indicates that this is irreversibly damaging. Hence the current methods
for removing laughing gas, as have been previously stated, have the
common feature that the gas stream is diluted with another
laughing-gas-free gas to a laughing-gas concentration of usually less
than 5%. In addition, measures are accordingly provided for the catalyst
to be able to operate isothermally, that is, to be cooled or heated.

[0009] The task of the present invention is based on a method for
improving of the sort mentioned at the beginning, so that a flexible
decomposition of laughing gas in a laughing-gas-bearing gas is possible,
without intensely increasing the temperature of the laughing-gas
decomposition and thus positively affecting the catalyst service life.

[0010] The present task is resolved with respect to method by the
combination of characteristics cited in the features of claim 1. Further
advantageous embodiments of the invention are cited in the subclaims.

[0011] According to the invention, the diluting gas, which is added to the
laughing-gas-bearing gas, is dry. Additionally, at least a part of the
exhaust from the catalytic decomposition of laughing gas is mixed into
with the diluted laughing-gas-bearing charge gas upstream of the
catalytic laughing-gas decomposition.

[0012] The invention considers any laughing-gas-bearing gas stream to be
within the scope of a laughing-gas-bearing gas. This can be, for one
thing, a laughing gas exhaled by a patient within the scope of a medical
application, or for another thing, an exhaust stream from production or
filling cylinders of laughing gas, for instance. The invention is suited
to treating any gas streams that contain laughing gas in any
concentration.

[0013] Through the method according to the invention, it is ensured that
the laughing-gas-bearing charge gas contains no or only a small portion
of water before supplying it for catalytic decomposition of the laughing
gas. The start-up temperature of catalytic laughing-gas decomposition
with a dry gas clearly lies below the start-up temperature for a gas that
contains water. This has the advantage, for one thing, that the charge
gas in the catalytic decomposition reaction does not have to be heated up
as much. For another thing, the temperature difference between the
start-up temperature and the maximum permissible temperature for the
catalyst or accessory compartments can be higher. Hence
laughing-gas-bearing charge gases in the catalytic decomposition can also
be processed with a higher share of laughing gas.

[0014] Through the dilution according to the invention of the
laughing-gas-bearing gas with dry diluting gas and the further mixing of
at least a part of the exhaust gas from the catalytic laughing-gas
decomposition, it is ensured that the exhaust gas from catalytic
laughing-gas decomposition is always nearly dry. By feeding back the
nearly dry exhaust from the catalytic decomposition of the laughing gas
and mixing it into the laughing-gas-bearing charge gas, the portion of
dry diluting gas needed is minimized. Thus very stable processing is made
possible. A part of the exhaust fed back is given off to the atmosphere
before mixing with the laughing-gas-bearing charge gas.

[0015] By diluting the laughing-gas-bearing charge gas according to the
invention with the dried diluting gas and mixing at least a part of the
exhaust from the catalytic decomposition of the laughing gas, a more
favorable and lower reaction temperature is used in the method according
to the invention with the same laughing-gas conversion as in prior art.
As a result, the materials selected are adapted to the lower reaction
temperature. Additionally, the method according to the invention allows
operation at the same conversion with a smaller reactor volume. This is a
particular advantage, together with the expanded spectrum of possible
materials, for application to laughing-gas decomposition in a clinic.
Through the method according to the invention, the decomposition of
laughing gas can be directly carried out on small and compact equipment
in a treatment space.

[0016] In accordance with the method according to the invention, the
moisture content in the total gas stream, which is passed as a charge
stream in the catalytic decomposition of the laughing gas, is held
constant by the diluting gas. By diluting the laughing-gas-bearing gas
with the dry diluting gas and by mixing at least a part of the exhaust,
the laughing-gas-bearing gas can itself also contain moisture up to
saturation. The charge stream for the catalytic decomposition of laughing
gas is always nearly dry. This leads to a stable reaction and therefore
to minimal measurement, control, and regulation expense. As a result, the
degree of the overall effect and therewith the economy of the method are
increased.

[0017] The advantages cited for the method according to the invention
occur in both isothermal and adiabatic reactions of catalytic
laughing-gas decomposition.

[0018] The flexibility of the catalytic decomposition of laughing gas
clearly increases through the method according to the invention. For one
thing, the breadth of variation related to a temperature increase is
clearly greater due to a greater share of laughing gas in the
laughing-gas-bearing charge gas as a result of reduced reaction
temperature with the method according to the invention. For another
thing, the concentration of the laughing gas in the laughing-gas-bearing
charge gas can be readily regulated before catalytic decomposition of the
laughing gas by supplying dry diluting gas. Through the combination
according to the invention of supplying dry diluting gas and mixing at
least a part of the exhaust from the catalytic decomposition of the
laughing gas, only a small amount of dry diluting gas is needed in the
method's normal operation. This amount can simply be increased for an
increased concentration of laughing gas in the laughing-gas-bearing
charge gas.

[0019] Advantageously, noble-metal catalysts are used as a catalyst on
carrier materials for the conversion according to the method according to
the invention. Palladium, rhodium, platinum, and ruthenium, for example,
number among the advantageous noble metals here, wherein palladium and
rhodium are especially preferred. Materials known in prior art known,
such as aluminum oxide, silicon oxide, or zeolite, for example, are
appropriately used as carrier materials.

[0020] Air or an inert gas is preferred as a diluting gas. Air is
cost-effective and available without limit. Just like the preferred inert
gases, air does not affect the reaction of catalytic laughing-gas
decomposition. Hence, air or an inert gas is outstandingly suited as a
diluting gas.

[0021] Preferably, the diluting gas is dried so that the
laughing-gas-bearing charge gas exhibits a dew point of -20° C.,
and especially preferably -40° C. A moisture content in the
laughing-gas-bearing charge gas that corresponds to a dew point of
-20° C., preferably -40° C., is sufficient to use the
method according to the invention to full advantage.

[0022] In one embodiment of the invention, the mixing of the exhaust takes
place in laughing-gas-bearing charge gas upstream of the heat-exchange
step and of heat exchange with the exhaust. In this embodiment of the
invention, the exhaust from the catalytic decomposition of the laughing
gas passes through the heat exchanger before it is mixed with the
laughing-gas-bearing charge gas. The diluted laughing-gas-bearing charge
stream is then passed with the mixed exhaust through the same heat
exchanger into the reactor for catalytic decomposition of the laughing
gas. Thus, in this embodiment of the invention, a heat exchanger is used
for cooling the exhaust and at the same time heating the
laughing-gas-bearing charge gas.

[0023] Alternatively, the hot exhaust can be passed at least partially
directly into the laughing-gas-bearing charge gas.

[0024] In another embodiment of the invention, the laughing-gas-bearing
charge gas is dried after supplying the diluting air to the
laughing-gas-bearing charge gas.

[0025] The drying of the diluting gas appropriately takes place with a
single-pass dryer, a molecular sieve, an adsorbent, an exchange-bed
adsorbent, or a diaphragm dryer. The means cited for drying the diluting
gas are established in prior art and present different advantages.
Single-pass dryers are an established and favorable means of drying a
charge gas. Through the method according to the invention, only a small
amount has to be dried in the diluting gas. Hence it is also sufficient
to replace the single-pass dryer at longer intervals. Adsorbents or
molecular sieves have to be regenerated. With a small amount of the
diluting gas to be dried, this regeneration is only required at longer
intervals. Exchange-bed adsorbents can be use to ensure continuous
operation of the method. In that case, an adsorbent is used for drying
the diluting gas, while at the same time a further adsorbent is
regenerated. In an advantageous embodiment, hot exhaust from the
catalytic decomposition of the laughing gas is used to regenerate the
adsorbent(s).

[0026] In another embodiment of the invention, the laughing-gas-bearing
gas can still be additionally supplied to a separate step before dilution
for drying or for cleaning out of halogen-bearing compounds.

[0027] Advantageously, the laughing-gas concentration in the
laughing-gas-bearing charge gas is measured before and/or after the
dilution with the diluting gas and/or right in front of the reactor. On
the basis of this measurement, the reaction temperature, the temperature
of the laughing-gas-bearing charge gas, the mixed amount of diluting gas,
and/or the mixed amount of exhaust are regulated. There can also
advantageously be a bypass valve for controlling the temperature of the
laughing-gas-bearing charge gas, in order to use the heat exchanger for
heat exchange between the laughing-gas-bearing charge gas and the hot
exhaust.

[0028] In one embodiment of the invention, several laughing-gas-bearing
charge streams are collected in a gas receptacle and from there passed as
a charge stream in the method according to the invention. This embodiment
of the invention is especially suited for production systems, tanks of
laughing gas, or cylinder filling. In such application cases, there is
for the most part exhaust piping for the laughing-gas-bearing gas
streams. In this embodiment of the invention, the existing piping is
connected to a low-pressure gas receptacle. All the laughing-gas-bearing
gas streams are collected in the low-pressure gas receptacle. Then the
laughing-gas-bearing gas is removed from the gas receptacle and is passed
as a charge stream in the method according to the invention. In this way,
fluctuations in the amount of an individual gas stream can be brought
under control by the gas receptacle.

[0029] As for the device, the task set is resolved through the
characteristic features of claim 6.

[0030] The device according to the invention for catalytic decomposition
of laughing gas includes a supply of laughing gas to a compressor, a heat
exchanger, a heater, and a fixed-bed reactor which exhibits a catalyst
and is suitable for catalytic decomposition of laughing gas, as well as
piping in the supply to the compressor. According to the invention, the
device further exhibits an engineered flow connection between the exhaust
outlet of the fixed-bed reactor and the supply to the compressor, which
leads through the heat exchanger. According to the invention, the piping
is connected to a dryer, which is suitable for drying a diluting gas.

[0031] With the present invention, it succeeds in particularly ensuring a
stable procedure. It can thereby react flexibly at different
concentrations of laughing gas in laughing-gas-bearing charge gas. The
start-up temperature and the reaction temperature of the catalytic
decomposition of laughing gas can, in contrast to a method according to
prior art, clearly be chosen to be lower.

[0032] The invention will be clarified in more detail in the following
using an embodiment example represented in the FIGURE.

[0033] It shows

[0034] FIG. 1 an embodiment example of the method according to the
invention and the device according to the invention.

[0035] FIG. 1 shows an embodiment example of the method according to the
invention and the device according to the invention for catalytic
decomposition 7 of laughing gas in a laughing-gas-bearing gas 1. The
laughing-gas-bearing gas 1 is diluted by means of a diluting gas 2.
According to the invention, the diluting gas 2 is nearly rid of its share
of water in the dryer 3. In this embodiment of the invention, air is used
as a diluting gas 2. After supplying the diluting gas 2 through the
piping 13, the exhaust 8 from the catalytic decomposition 7 is mixed 4
with the laughing-gas-bearing charge gas 12. After mixing 4 the exhaust 8
from the catalytic decomposition 7 of the laughing gas, the
laughing-gas-bearing charge gas 12 is compressed 5, and passed into the
heat exchanger 6. In the heat exchanger 6, the laughing-gas-bearing
charge gas 12 is heated by heat exchange with the exhaust 8. The exhaust
8 is cooled off at the same time in the heat exchanger 6. The heated,
laughing-gas-bearing charge gas 12 is passed through a further, optional
heater 11 as a charge in the catalytic decomposition 7 of the laughing
gas. The heater 11 is used only for additional, optional heating of the
laughing-gas-bearing charge gas 12. The inlet temperature in the
catalytic decomposition of laughing gas in this embodiment of the
invention is roughly at 350° C. The catalytic decomposition of the
laughing gas hereby occurs at atmospheric pressure or a little higher
pressure. A part of the exhaust 8 is taken out of the process. The
exhaust 8 is further cooled by a further, optional heat exchanger 10.
Through the additional cooling by means of the heat exchanger 10, it is
ensured that the next pieces of equipment, such as the compressor, are
protected from overheating. The embodiment example represented is suited
for use in a treatment space and for the decomposition of laughing gas
from a medical treatment. Through the heat exchanger 10, the permissible
temperature for the part of the exhaust 8 that is given off 9 to the
atmosphere can also be monitored here. For optimal control of the inlet
temperature of the laughing-gas-bearing charge gas 12 in the catalytic
decomposition 7 of laughing gas, at least a part of the
laughing-gas-bearing charge gas 12 can be passed on to the heat exchanger
6 with a bypass valve 14.